1. Field of Invention
The present invention relates to a method of manufacturing a flash memory. More particularly, the present invention relates to a method of manufacturing a flash memory having a dual floating gate structure.
2. Description of Related Art
A conventional flash memory is a type of erasable programmable read-only memory (EPROM), which in turn is a type of non-volatile memory. In general, an EPROM cell comprises two gates. One of the gates, known as a floating gate, is fabricated from polysilicon and is used for charge storage. The second gate, known as the control gate, is used for controlling the input and output of data. The floating gate is located beneath the control gate, and is generally in a floating state because there is no connection with external circuits. The control gate is normally wired to the word line. One of the advantages of flash memory is its capacity for block-by-block memory erasure. Furthermore, the speed of memory erase is fast, and normally takes just 1 to 2 seconds for the complete removal of a whole block of memory. For most other EPROM, memory erasure can take up to several minutes due to its bit-by-bit operation.
FIG. 1 is schematic, cross-sectional views of a conventional flash memory. The conventional flash memory is constructed with a floating gate transistor. A tunneling oxide layer 102 is formed on a P-type substrate 100. A floating gate 104 is formed on the tunneling oxide layer 102 and a control gate 108 is formed over the floating gate 104. A dielectric layer 106 is formed between the floating gate 104 and the control gate 108 and encloses the floating gate 104. Therefore, the floating gate 104 is electrically isolated from the environment. Additionally, an N-type drain region and an N-type source region 112 are respectively formed in the substrate 100 adjacent to a portion of the substrate 100 beneath the tunneling oxide layer 102.
Basing on the Flowler-Nordheim effect, during data storage, a voltage of about 8V is applied between the drain region 110 and the source region 112 and a relatively high voltage is applied on the control gate 108. Hence, the hot electrons flow from the source region 112 into the floating gate 104 and penetrate through the tunneling oxide layer 102 near the drain region 110. Therefore, the critical voltage of the floating gate transistor is increased and the object of data storage is achieved.
While erasing the data stored in the flash memory, a proper negative voltage is applied on the control gate 108. Therefore, the electrons captured in the floating gate 108 escape from the floating gate 108 and penetrate through the tunneling oxide layer 102. After that, the data previously stored in the flash memory are erased and the floating gate transistor is restored to the state before the data were stored in the flash memory.
At this time, each flash memory cell can store only one bit. In order to meets storage requirements, that is, to enlarge the numbers of bits stored in a unit area of a flash memory cell, the typical flash memory manufacturing technique is to increase the numbers of bits stored in the unit area of a flash memory.